Carboxy polysaccharides, such as hyaluronic acid and alginic acid, have excellent water solubility together with moderate viscosity, adhesiveness, moisture-retaining properties, and biocompatibility, and thus have been widely used as food additives, medical materials, and additives for cosmetics and commodities, particularly as thickening materials.
Of such carboxy polysaccharides, a common, widely used compound is hyaluronic acid. Hyaluronic acid is a water-soluble polymer, and, for its excellent dispersibility and water-retaining properties, has been used in various fields, including food products, cosmetics, drugs, etc. Further, hyaluronic acid has high safety, and thus has been used, in the medical field, as a raw material for joint lubricants, adhesion barriers, etc.
The formation of an insoluble derivative by modifying carboxyl groups of hyaluronic acid has been known. For example, Patent Document 1 describes a water-insoluble derivative of a polyanionic polysaccharide obtained by mixing a polyanionic polysaccharide containing hyaluronic acid, a nucleophilic agent, and an activator in an aqueous mixture, as well as a method for producing the same.
Further, Patent Document 2 describes a hyaluronic acid amide, a derivative thereof, and a method for producing the same. However, the provision of an injectable gel having high viscoelasticity as in the present invention is nowhere mentioned.
Further, Patent Document 3 discloses a natural wound-healing product containing a polysaccharide polymer and one fatty acid chain attached to the polysaccharide polymer, which undergoes a reversible change of state from a gel to a solution.
However, polysaccharides disclosed in these documents have poor viscoelasticity in an aqueous solution, and it is difficult to use them as hydrogels that can be injected into the body through a small tube like an injection needle.
Further, Patent Document 4 states that a product obtained by modifying carboxymethylcellulose side chains with phosphatidylethanolamine can be used as an adhesion barrier agent, and Patent Document 5 states that a product obtained by modifying hyaluronic acid side chains with phosphatidylethanolamine can be used as an adhesion barrier agent.